1. Field of the Invention
The present invention relates to a surveying instrument and more particularly to an electro-optical distance measuring device.
2. Description of the Prior Art
In the art there are known many kinds of electro-optical distance measuring devices of the type in which the objective lens of the collimating optical system and the objective lens of the distance measuring optical system are used in common to the two systems. In this type of known devices the distance measuring optical system needs a very precise alignment of the optical paths for transmission and reception to obtain high accuracy of measurement. Because of it there have been great difficulties not only in design and manufacture but also in assembling and adjustment. As for adjustment it may be carried out more easily by providing separate adjustment means proper to the respective optical systems. However, it renders the construction of the whole device complicated and the size larger.
As to means for dividing the optical path into that for collimating optical system and that for distance measuring optical system there have already been realized also many types of optical path-dividers (beam splitters). For the known electro-optical distance measuring devices of the type in which one half of the objective lens aperture is used to transmit the light and the other half is used to receive the light, there has been generally employed a slant half-mirror to branch the optical path for distance measuring optical system. In this case, the half-mirror must be disposed 45.degree. inclined relative to the optical path. Therefore, the thickness on optical axis of the prism constituting the half-mirror is large, which in turn lengthens the optical path of the collimating optical system. As a whole the distance measuring device has a large structure.
Since the light generally used for distance measurement is infrared, it is preferable to form the optical path-divider using a dichroic mirror. However, the use of dichroic mirror involves some problems. Because the dichroic mirror is usually formed of a multi-layer thin film, its performance drops down with increasing the incident angle of the infrared light. When the incident angle is smaller, the dichroic mirror exhibits better performance and it is easier to manufacture. Considering these points Japanese Laid Open Patent Application No. 86,771/1982 has proposed an arrangement of electro-optical distance measuring device in which the angle of incidence to the dichroic mirror is smaller than 45.degree.. However, even in this device, the prism of optical path-divider has still large thickness on optical axis, which is against the desire to reduce the size of the collimating optical system.